Our objective is to resolve the regulation of nitrogenase, nitrate reductase, and nitrite reductase in relation to the ammonia utilizing enzymes glutamine synthetase, glutamate, synthase, and glutamate dehydrogenase. Nitrate and nitrite reductase and glutamine synthetase have been purified to electrophoretic homogeneity. A characterization of effectors of these enzymes is in progress. Activity levels of these enzymes in response to different nitrogen sources have been determined. Over 2000 mutants with phenotypes indicating lesions in nitrogen metabolism have been isolated. A technique using polyvinylpyrrolidone as cryopreservative has been developed for the long-term storage of blue-green algal mutants. Preliminary characterization indicates mutants with phenotypic lesions all of the enzymes under study except glutamate dehydrogenase.